1. Field of the Invention
The present invention relates to external cavity lasers with intra-cavity frequency doubling.
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2. Background Art
Volume Holographic Gratings (VHGs) also known as Volume Bragg Gratings (VBG) or Bragg mirrors are produced by a holographic process. Holography is the process of recording phase information into a material that is sensitive to the intensity of the incident illumination (“Introduction to Fourier Optics”, J. W. Goodman, McGraw-Hill, 1968). Early materials were primarily photographic films, but modern photorefractive materials additionally include dichromated gelatin films, LiNbO3 and other crystals, polymers, and glasses. Amplitude and phase information can be recorded through the interference of mutually coherent signal and reference beams.
When the signal and reference beams are simple plane waves, the material records the single sinusoidal intensity pattern formed by their interference. The grating is referred to by its grating vector (normal to the plane of constant index), which has a grating magnitude and orientation. The magnitude is the refractive index modulation depth for materials that contain phase gratings, and is the absorption modulation for materials that contain amplitude gratings. The orientation is determined by the angle between the recording beams and the recording material. If the signal or reference beam, or both, are not simple plane waves but rather carry information in the form of phase or intensity variations, then the recorded hologram can be thought of as being composed of many superimposed individual gratings each recorded by pairs of plane waves from the Fourier decomposition of the recording beams. A description of this process is found in the reference by J. W. Goodman noted above.
Holographic recording can be used with thin or thick media. When the material in which the hologram is present is thick, then Bragg selectivity occurs (“Coupled Wave Theory for Thick Hologram Gratings”, H. Kogelnik, The Bell System Tech. J. 48:9, 1969). Volume hologram reflection gratings have been shown to be an extremely accurate and temperature-stable means of filtering a narrow passband of light from a broadband spectrum. This technology has been demonstrated in practical applications where narrow full-width-at-half-maximum (FWHM) passbands are required. Furthermore, such filters have arbitrarily selectable wavefront curvatures, center wavelengths, and output beam directions.
Others have described VHGs as output coupler of an external cavity laser (U.S. Pat. No. 5,691,989). In the case of an external cavity laser with intra-cavity frequency doubling, the VHG can also be used as output coupler of a laser cavity (US 2006/0029120; US 7,248,618). These patents describe a method by which a non-linear material such as a periodic poled lithium niobate (ppln) or a periodically poled lithium tantalite (pplt) is inserted in between a laser diode and a reflective VHG which acts as the output coupler. The second harmonic light generated by the ppln or pplt, via the so-called intra-cavity frequency doubling, passes through the output coupler VHG. Because the optical loss of the VHG, caused by absorption and scattering at the second harmonic can be quite high (20% or more), it is desirable to have an output coupler VHG design that prevents the second harmonic light from passing through the VHG.